A power transformer is used to step up, or step down, a voltage. That is, the output voltage is the input voltage times a step factor. The step factor is generally called the transformer ratio and is also known as turns ratio. This ratio is defined as the ratio of the primary and secondary voltages for a two winding transformer. Generally, a transformer that steps up voltage will step down current while a transformer that steps down voltage will step up current. Since power equals voltage times current, power is the same on both sides of the transformer, ignoring transformer losses. Aside from being multiplied by the transformer ratio, the current entering a power transformer should be equal to the current leaving the power transformer. A difference between the current entering the transformer and the current leaving the transformer can indicate a fault within the transformer where current is being diverted into the transformer rather than passing through the transformer.
Differential protection of power transformers is a technique that compares the current entering the transformer with the current leaving the transformer. One side of a transformer is the primary while the other side is the secondary. Usually, the side where power enters is the primary. A differential protection system senses the current at the primary side and also senses the current at the secondary side. The area between the current sensors is called the protection zone. The differential protection system determines if there is an excessive difference, aside from the scaling factor, between the two current sensors. If the difference exceeds the relay setting (differential threshold), a fault within the protection zone is likely, and a protection relay initiates operation of a circuit breaker or other device to isolate the power transformer.
The difference between the primary current and the secondary current that is required to trip the differential protection system and to cause the protection relay to operate can be called the differential threshold of the differential protection system. A differential protection system with a lower differential threshold may be more sensitive, but can be falsely triggered by non fault events.
Generally, current transformers are used in a differential protection system to sense the primary current and the secondary current. Current transformers typically have iron cores and may saturate. Transformer core saturation occurs when more magnetic flux is induced within the transformer than can be handled by the core. When a transformer core saturates, it may lose its inductive characteristics allowing currents in the transformer windings to temporarily spike to extremely high levels. Unequal saturation between the current transformer sensing the primary current and the current transformer sensing the secondary current is an example of a false triggering event as no fault is involved.
To avoid erroneously detecting fault conditions for reasons such as current sensor saturation, the differential threshold of a protection system may be set as a percentage of the current passing through the transformer. This differential threshold setting generally is provided by adding restraint components to stabilize the protection relay. Relay stabilization improves performance since high through currents will require a higher differential current to operate the relay. This quality generally is characterized by the slope of the relay. The slope is given by a sloped line that relates the current passing through the transformer with the differential threshold setting. The line is sloped because a higher through-current implies setting a higher differential threshold. Such a relationship may be illustrated using an upward sloping line when plotted on a graph with through-current on the horizontal axis and differential current threshold on the vertical axis. In systems where the relay is controlled digitally, multiple slopes may be utilized to avoid inappropriately operating the protection relay in conditions involving severe current transformer saturation caused by high fault currents.
Inrush current is the input current drawn by a device when power is initially applied to the device. Inrush current is a startup transient. When a power transformer is first energized, an inrush current much larger than the rated transformer current can flow for up to tens of seconds. That is, when a transformer is first powered on, a higher current must flow into the transformer to establish the magnetic fields within the transformer core. Inrush current flows at the energizing side of the transformer, while there may be little or no current flow at the other side of transformer. In a networked application, the energizing side of the transformer may be the primary side or the secondary side. Since inrush current generally flows on only one side of the transformer, the current differential between the primary and secondary sides of the transformer easily can exceed the differential threshold of the differential protection system and can cause the protection relay to isolate the power transformer even though an actual fault does not exist.
The unnecessary isolation of a power transformer during an inrush condition can be mitigated by detecting the inrush current and, in response, blocking the differential protection element within the protection relay. Traditionally, inrush current is detected by extracting the second harmonic component from the inrush current using mathematical algorithms. This technique typically involves applying filters to the current measurements to isolate the second harmonic component which is a portion of the current signal at about twice the operating frequency of the power system. Various filter designs, of differing complexities, have been developed to extract this second harmonic information. Varying response characteristics at this second harmonic for different transformers, as well as the complexity of the related filter designs, often complicate such traditional approaches.
Accordingly, there is a need in the art for a power transformer differential protection system for more accurately detecting inrush current and to reduce the unnecessary isolation of the power transformer during an inrush condition.